Extend checking correctness of bounds declarations using equality of variables

Build on work item #207 to include variable equality.   This involves two work items:
- Use equality facts just inferred from the computation done by an expression (for example, `a = b`).
- Doing a dataflow analysis of a function body to infer equality facts.  For example, in 
```
if (a == b) {
   lhs;
} else {
  rhs;
}
```
we can refer that `a == b` in `lhs`.
